If a galaxy followed strict Keplerian expectations based only on visible matter, what should happen to orbital velocity far from the center?
Answer
Velocity should decrease following the square root of the distance.
Following Kepler's laws, similar to the Solar System where mass is centralized, the orbital velocity is expected to fall off as the speed decreases with the square root of the distance once past the bulk of the visible mass.
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Astrophysics: Galaxy Rotation Curves and Dark Matter - YouTube
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